Pentax 6x7 TTL Prism Repair and Exposure Meter Adjustment

The TTL Prism on my Pentax 6x7 camera has a built in exposure meter that was starting to go a bit whacky. The needle would jump all over at random and the view finder area would go black if I tried to use the camera in any position other than the default horizontal position. And it was consistently under exposing everything by 1-1/2 stops.




I decided I would have a go at repairing it and document what I did and hopefully some other folks with similar problems will find this helpful and be able to repair their prism meters also.

First thing you need to do is get the top of the prism off. You will need to have a spanner wrench type tool for removing the top cap screw of the shutter speed dial. I have one of these. I am sure some of you creative folks will make something else work, that is fine.

There are 8 small screws around the outside that you need to remove. 2 of the screws are behind the front name plate so that must be gently pried off to get those 2 screws out.

Here is the front name plate removed and you can see where the 2 screws are located

Gently pry off the front name plate to remove the 2 screws holding the top on

After you remove the 8 screws and the shutter speed dial you can gently tap the edges of the prism top with something soft like a plastic screwdriver handle to get the top to slowly come off.

NOTE: You do not need to remove the on/off button switch to take the prism top off

You should end with this

Note the shutter speed dial spring and washers, don't lose them!

Adjusting the exposure meter when it is off a bit is not to hard, there is one big adjustment screw you can use to correct the exposure needle alignment.

Here you can see the large plastic adjustment screw. I only needed to adjust mine a couple millimeters clockwise to correct the 1-1/2 stop underexposure. Look closely and you can see the black felt pen marks I used to mark the original position and how it is now turned just slightly clockwise.

Large "POTS" adjustment screw for the built in exposure meter

 
I had 2 more problems though, the meter would go black when I tilted the camera and also the needle was very erratic. Turns out the erratic needle was a loose +6v cable on one of the prism contacts and the meter going black was from the reflecting prism being loose inside the small slot it sits in.

Here you can see the +6v wire that has come loose

Loose +6v wire

Basically I just soldered on another piece of wire and sealed it with some shrink wrap

This next photo is not very good but it is trying to show you the small refective viewfinder prism the meter uses when you look thru the viewfinder. I used some contact cement to glu it back into position. It is the rectangle glass looking thing between the two electrical posts at the bottom of the viewfinder opening.

Here is the top of the meter with the new wire soldered in place and all tucked in ready for assembly. I still need to adjust the exposure first.

Notice the exposure meter and needle assembly on the left

To adjust the exposure meter I just manually triggered the on/off switch with my hand and made several test exposures with different ISO, aperture and speed settings. I used my DSLR in center weighted mode and my Sekonic Auto Studio II light meter as the test metric devices.

Testing the exposure settings

After thorough testing with different ISO, aperture and shutter settings in both bright daylight and dim light at night I feel I have it calibrated it about as good as it can get. It is almost spot on with my DSLR and 90% spot on with my Sekonic. It is one happy prism and exposure meter now!

Happy Prism and exposure meter!

I hope this post will be helpful to anyone else looking to repair their Pentax 6x7 TTL Prism